Now as MySQL 5.0 is finally released as GA, or stable I guess many of your are asking yourself if it is the time to upgrade. Let me express my opinion on this matter.

First I guess most of you running live application would follow "do not fix what is not broken" rule, and I think this is right. I know number of installations still running MySQL 3.23 quite successfully and even larger number of people happy with 4.0.

I would not stay with 3.23 until now as it is barely supported - it might work well for you still but if you run into some issue it will unlikely be fixed in 3.23 and you might be forced to upgrade to 4.0 while you're not ready. You will also likely find a lot of gotchas in this release, which are often already erased from developers memory, also most mailing/forum members are running higher versions.

With MySQL 5.0 being released I expect limited fixes will be done in 4.0 so this is release I'd try to move from in the next 6 months or so, possibly to 4.1 first. This would leave you with more mature version, and also upgrade to next version is much better tested. Do not expect this upgrade to be fully painless however - there are some sorting order changes, timestamp format changes and other nasty surprises. Performance may be affected one way or another, but in general unless you can get advantage of new MySQL 4.1 optimizations/features it will be few percent lower. Character sets is one more story - if you're not using same latin1 charset everywhere you need to be careful and watch which charset is used for input data and if it matches the encoding you supply it in. Also do not be mistaken - Unicode (utf8) is slower and taking more space than single byte character sets - especially if you're moving from win1251 or other previously single byte encoding to utf8 you may be surprised with increased database size and resource consumption.

So What is about MySQL 5.0 ? My personal recommendation would be to wait in production applications, unless you really need some new 5.0 features.